The investigation of strong-field quantum electrodynamics (SF-QED) processes requires extreme field intensities approaching the Schwinger limit^[1^], which cannot be reached by state-of-the-art laser facilities. However, when a relativistic electron travels through an existing intense laser field, the field experienced by the electron in its rest frame is Lorentz transformed to a level where SF-QED effects matter. Therefore, relativistic electrons can act as probes to test these effects. This interaction is known as Thomson or Compton scattering, depending on whether the process is elastic or inelastic scattering. During the scattering, collimated X-/gamma-rays are generated and can be used as radiation sources, commonly known as the inverse Compton scattering (ICS) source. Previously, scattering experiments were conducted in the laboratories of particle accelerators, but the remarkable advances in laser wakefield acceleration (LWFA)^[2^] have now enabled the study of these experiments under the all-optical setup within high-intensity laser laboratories. The principle of the scattering process is shown in Figure 1.

There are two possible configurations for all-optical scattering experiments. One is a simplified version that involves only one laser beam. This laser beam first drives a wakefield accelerator, and then its leftover energy is reflected by a plasma mirror^[3^–10^] onto electrons, causing head-on scattering. The other scheme features a layout with two independently tunable laser pulses^[5^,11^–18^]. Although precise temporal and spatial synchronization between the two beamlines is technically difficult, the dual-beam scheme offers the distinct advantage of independently controlling the properties of the scattering process. Each of the two lasers can be individually tuned to optimal conditions, enabling the generation of high-energy electrons and a scattering laser that meets specific requirements, respectively. Specifically, by keeping the parameters of the LWFA constant, one can exclusively manipulate the colliding pulse, enabling a single-variable investigation into the scattering process.

To investigate these scattering processes, we established an experimental platform for dual-beam all-optical Thomson/Compton scattering (EPATCS) with versatile parameter tuning capabilities. The EPATCS is a tabletop electron–photon interaction setup, where high-energy electrons are accelerated by one laser beam and then collide with the second laser beam. The flexibility in parameter tuning not only enhances control over experimental conditions, but also opens up new prospects for investigating high-field scattering processes. By precisely controlling the intensity, polarization state and field mode of the colliding laser beam, we can generate X-/gamma-rays with varying characteristics, which can serve as light sources for applications such as high-density dynamic imaging^[19^–26^] and vortex photons for photonuclear physics^[27^,28^]. The EPATCS can also be built on PW laser facilities and provides the possibility of interaction between multi-GeV electrons and high laser intensity under extreme conditions^[18^] to study SF-QED effects, including the radiation reaction^[11^,12^], pair production^[10^,29^–31^], conversion of angular momentum transfer in the collision process^[32^–34^], etc.

This paper is organized as follows. In Section 2, we discuss the versatile tuning of ICS X-/gamma-ray source parameters and their impact on enhancing control over high-field processes and tunable radiation spectra. Section 3 presents the experimental results based on the EPATCS at the 200 TW laser of Shanghai Jiao Tong University. In Section 4, we elucidate the prospective SF-QED research of the EPATCS.

The EPATCS offers the potential for multi-parameter adjustments, including tuning the electron beam energy, altering the collision angle between electrons and photons and adjusting the parameters of the colliding laser (such as the laser intensity, wavelength, polarization and orbital angular momentum). In this section, we introduce the experimental designs and explorations to investigate the adjustable parameters and their effects on radiation.

A schematic diagram of a dual-beam collision layout is as shown in Figure 2(a), to investigate the influence of multiple parameters on the photon energy of radiation in the ICS process. The combination of the optical system and guide rail enables precise adjustments to the electron–laser collision angle. Adjustments in the collision angle $\varphi$ , the focusing intensity ${a}_0$ , the wavelength $\lambda$ , the polarization state $P$ and the orbital angular momentum $\overrightarrow{L}$ of the colliding beam will significantly alter the characteristics of the radiation emitted. Adjusting the collision angle $\varphi$ results in a significant variation in the energy spectrum of the X-/gamma-ray from tens of keV to MeV^[35^]. When the normalized vector potential of the colliding laser, ${a}_0$ , varies from less than 1 to greater than 1, the electron–photon interaction shifts from a linear to a nonlinear process^[13^,16^,17^]. Modifying the wavelength $\lambda$ alters the colliding laser photon energy ${E}_0=\mathrm{\hslash}c/\lambda$ , which in turn modifies the energy of the scattered photons, that is, the radiation energy^[15^]. Since this is a fundamental model of the scattering process, the polarization state of the colliding laser directly influences the polarization of the emitted radiation^[9^]. When the colliding laser is transformed from a Gaussian laser to a Laguerre–Gaussian laser, adjustments in the orbital angular momentum directly affect the orbital angular momentum of the X/gamma radiation^[32^,36^].

To illustrate this principle with a specific example, adjusting the collision angle demonstrates how changes in laser parameters can regulate radiation. In the ICS process, altering the collision angle between electrons and photons can significantly modify the energy spectrum of the emitted radiation. The energy of the emitted photon ${E}_\mathrm{sc}$ can be expressed by the following^[37^]:where $n$ represents the nonlinear order, ${E}_0$ represents the incident photon energy, ${\gamma}_\mathrm{e}$ denotes the Lorentz factor of an electron, ${a}_0={eA}_0/{m}_\mathrm{e}c$ is the normalized amplitude of the vector potential and $e$ and ${m}_\mathrm{e}$ are the elementary charge and the rest mass of the electron, respectively. According to Equation (1), the energy of the scattered photons is correlated with that of the incident photons in the following manner: ${E}_\mathrm{sc}\sim {E}_0{\gamma}_\mathrm{e}^2\eta$ , where $\eta =1-\cos \varphi$ is a parameter related to the collision angle, exhibiting a clear range from 0 to 1. Therefore, altering the collision angle acts as an effective approach to enable the continuous adjustment of the ICS radiation energy range. Figure 2(b) illustrates the radiation energy at the observation angle of $\theta =0^{\circ}$ generated by linear ICS at various collision angles for electron energies ranging from 100 to 300 MeV.

In Section 3, the collision angle $\varphi$ and the polarization state of the colliding laser are altered to validate the feasibility of this platform. In Section 3.2, the ICS processes are experimentally validated at collision angles of $30{}^{\circ}$ and $135{}^{\circ}$ , corresponding to the theoretical values calculated in Figure 2(b), and the corresponding radiation energy spectra are diagnosed and analyzed.

To validate the applicability of the EPATCS, it was initially integrated into the 200 TW Ti:sapphire laser facility at the Key Laboratory for Laser Plasmas of Shanghai Jiao Tong University. A 5 J p-polarized laser pulse with a duration of $\tau =25$ fs (full width at half maximum, FWHM) can be delivered to the target. A schematic diagram of the experimental setup and its primary components are referenced in Appendix B.

The laser pulse for LWFA is focused by an off-axis parabolic (OAP) mirror of F# 20 to a Gaussian-like spot size, containing 30% energy with an FWHM diameter of 26 μm, and the focused laser intensity can reach up to $5\times {10}^{18}\ \mathrm{W}\cdot {\mathrm{cm}}^{-2}$ , corresponding to a normalized vector potential of ${a}_0=2$ . The laser was focused above a supersonic gas jet of nitrogen. The electron energy can be effectively adjusted by modifying the relative position of the nozzle and the laser focus and adjusting the plasma density.

As shown in Figure 3, consistent energy electron beams, tunable within the range of 200–600 MeV, can be achieved under stable conditions. For each scenario, the plasma density and acceleration length are indicated, resulting in relatively stable electron energy spectra. Under appropriate plasma density, decreasing the acceleration length within a certain range can effectively enhance the energy of relativistic electrons. The corresponding electron spectrum is shown in Figures 3(a)–3(d). However, as shown in Figures 3(d) and 3(e), when the plasma density is significantly increased, the phase velocity of the wakefield decreases, causing the electrons to dephase earlier, leading to a reduction in their energy. Based on the scale shown in Figure 3 and the actual distance of the image plate from the target, which is 1.6 m, the transverse divergence angle of the electron bunch is approximately 3 mrad (FWHM).

The tuning energy of the electron enables the radiation energy spectrum to span a wide range. As indicated in Equation (1), when ${a}_0\ll 1$ (linear scattering regime), the energy of the emitted photon ${E}_\mathrm{sc}$ is directly proportional to the square of the electron Lorentz factor ${\gamma}_\mathrm{e}$ , a relationship that has been verified in several laboratories^[13^,16^,38^]. Figure 2(b) demonstrates that under a fixed collision angle, the radiation energy spectrum varies accordingly with changes in electron energy.

In the scattering experiments, the relativistic electrons generated by LWFA have a cut-off energy of approximately 300 MeV, approaching a continuous spectrum. Two experimental configurations with collision angles of $\varphi =30{}^{\circ}$ and $\varphi =135{}^{\circ}$ are set, as shown in Figures 4(a) and 4(b), respectively. The colliding beam is derived from the main laser pulse using a small pick-up mirror and is focused to a spot with an 8 μm FWHM. With a pulse energy of 200 mJ and a pulse duration of 25 fs, the on-target peak intensity is approximately $4.8\times {10}^{18}\ \mathrm{W}\cdot {\mathrm{cm}}^{-2}$ . The range of the radiation spectrum can be determined by altering the collision angle using Equation (1). Metal filter sheets of various materials and thicknesses are designed for radiation diagnosis.

In the experimental setup with a collision angle of $30{}^{\circ}$ , Ross filter pairs were selected on the basis of the K-absorption edges of metal filters with varying materials and thicknesses. The signal on the image plate behind the filters was obtained from a single shot. Using the least-squares method, we obtained convergence results and derived the energy spectrum. As shown in Figure 4(c), the quasi-monoenergetic peak of the X-ray spectrum at around 65 keV might come from ICS involving only a subset of electrons. When the electron beam interacts with the laser beam at a small angle, their velocities become comparable, causing the laser pulse to continuously interact with the same portion of the electrons within the Rayleigh length. This consequently results in the production of quasi-monochromatic X-rays.

In the case of a collision angle of $135{}^{\circ}$ , the radiation energy spectrum extends beyond 1 MeV. The intensity distribution of the high-resolution CsI fluorescence from a single shot, located behind the filters, was captured by a 16-bit Andor electron-multiplying charge-coupled device. The iterative least-squares method is adopted for the numerical analysis of the transmission coefficient^[39^] to calculate the final energy spectrum of the X-rays, as illustrated in Figure 4(d). Variations in the collision angle impact the radiation energy spectrum, which is directly influenced by the electron energy spectrum. The basic principles of spectrum diagnostics can be referred to in Appendix A.

Polarized X-rays can be used to probe the characteristics of magnetic structures in structural magnetism and distinguish between chiral and helical magnetic structures^[40^–44^]. Based on the EPATCS, we generate polarized X-rays via the collision of a polarized laser beam and the electrons. As shown in Figure 5(a), a half-wave plate or a quarter-wave plate is incorporated into the scattering laser path to change the polarization state of the scattering laser, which in turn modifies the polarization state of the generated radiation. An OAP mirror of F# 5 is used to focus the colliding laser. Under linear polarization conditions, the focal spot diameter and laser intensity are the same as those presented in Section 3.2, whereas under circular polarization, the laser intensity is approximately $2.4\times {10}^{18}\ \mathrm{W}\cdot {\mathrm{cm}}^{-2}$ . The collision angle is $135{}^{\circ}$ and the experimental layout is as illustrated in Figure 5(a).

As shown in Figure 5(a), a cylindrical polyethylene converter with a diameter of 2 cm and a length of 15 cm is placed 1.9 m from the impact point, and four image plates are placed around the converter. The secondary photon signal radiated on the scatterer is diagnosed in the vertical direction of X-ray propagation based on the Compton scattering. When the scattering angle is close to $90{}^{\circ}$ , the azimuth distribution of scattered photons is highly dependent on X-ray polarization, making Compton scattering effective for the diagnosis of polarization^[45^]. The expressions of the scattering cross-section of Compton scattering in the vertical direction of propagation for linear polarization (see Equation (2)) and circular polarization (see Equation (3)) are as follows^[46^]:where ${r}_\mathrm{e}$ represents the classical radius of the electron, where ${\varepsilon}_0$ is the energy of the incident photon, $\varepsilon$ is the energy of the scattered photon and ${\theta}^{\prime }$ denotes the scattering azimuth angle. According to Equation (2), Compton scattering of linearly polarized X-rays through the scatterer in the vertical direction is a function of the azimuth angle ${\theta}^{\prime }$ . When the incident X-ray is circularly polarized, Equation (3) reveals that the vertical Compton scattering is independent of the azimuth angle ${\theta}^{\prime }$ . In the experiment, the optical axis of the half-wave plate is adjusted to an angle of $22.5{}^{\circ}$ from the original horizontal polarization direction, thus producing a linear polarization angle of $45{}^{\circ}$ from the horizontal direction, as shown in Figure 5(b). The reason behind this choice of angle is to facilitate differentiation of the background signal. Due to the wide spectral width of the laser and the bandwidth limitations of the quarter-wave plate, the conversion efficiency of linearly polarized light to circularly polarized light is approximately 80%.

We conducted simulations using FLUKA software to simulate the process; the results are shown in Figures 5(c) and 5(d) and the experimental results are shown in Figures 5(e) and 5(f). Each result accumulated 100 shots, which facilitated the diagnosis of the polarization characteristics of the radiation through the scattered electron distribution. The four image plates are arranged to correspond to azimuth angles ranging from $-225{}^{\circ}$ to $135{}^{\circ}$ , with the portion corresponding to $100{}^{\circ}$ – $135{}^{\circ}$ being absent, due to constraints on the size of the image plate. Figures 5(g) and 5(h) include both theoretical simulation results and experimental data, with normalized intensity signals. The linear polarization diagnostics show a periodic intensity distribution across the four image plates, indicative of a Compton scattering signal corresponding to the scattering of linearly polarized X-rays. The generated linearly polarized radiation spectrum is illustrated in Figure 4(b). The degree of polarization $\left({I}_{\mathrm{max}}-{I}_{\mathrm{min}}\right)/\left({I}_{\mathrm{max}}+{I}_{\mathrm{min}}\right)$ according to the experimental results is about 0.42, and the background greatly influences this result. Moreover, the diagnostic results of circularly polarized X-rays exhibit uniformly distributed intensity signals overall, although a weak periodic intensity distribution signal is still observed near $-180{}^{\circ}$ . This suggests that the degree of polarization of circularly polarized X-rays generated by the ICS is not 100%, but rather an elliptical polarization state along the transverse axis. This result matches the effect of the waveplate bandwidth described above. Our results show that the X-ray diagnostics for circular polarization lack full symmetry. This asymmetry arises from the off-center positioning of the polyethylene scatterer during X-ray irradiation^[9^].

The aforementioned experimental results demonstrate precise control over electron energy, with a wide range of radiation energy and polarization states. These verify the versatile multiparameter tunability of the EPATCS. In our platform, the pulse duration of the electron bunch generated by LWFA is typically a few femtoseconds^[47^], and the laser pulse duration is 25 fs. Therefore, in order to realize the collision process described in Sections 3.2 and 3.3, following the approaches outlined in previous works^[48^,49^], we implemented spatial and temporal synchronization, achieved measured time jitters below 10 fs and maintained a high spatial accuracy of electron–photon collisions within 5 μm. Moreover, our experimental results confirm that this level of precision is essential for stable electron–laser collisions.

By integrating with multi-PW laser facilities, the adjustable parameters of the EPATCS provide a fundamental platform for the approaching research initiatives, including multiphoton Thomson/Compton scattering, radiation reaction, vacuum polarization effects and pair production^[45^,50^–55^]. When high-energy electrons interact with an intense laser, and the laser intensity in the rest frame of the electron approaches or reaches the Schwinger critical field ${E}_{\mathrm{s}}={m}_\mathrm{e}^2{c}^3/e\mathrm{\hslash}$ , the relativistic and nonlinear effects of the electrons in the electric field become significant. To describe such quantum effects, the quantum nonlinearity parameter ${\chi}_\mathrm{e}=\sqrt{{\left({F}^{\mu \nu}{p}_{\mu}\right)}^2}/\left({E}_{\mathrm{s}}{m}_\mathrm{e}c\right)$ is defined, where ${F}^{\mu \nu}={\partial}_{\mu }{A}_{\nu }-{\partial}_{\nu }{A}_{\mu }$ is the four-tensor electromagnetic field and ${p}_{\mu }$ represents the particle’s four-momentum. When ${\chi}_\mathrm{e}\ll 1$ , classical electrodynamics can effectively describe the interaction between particles and electromagnetic fields. When ${\chi}_{\mathrm{e}}\gtrsim 1$ , quantum effects (such as electron–positron pair production and nonlinear Compton scattering) become significant, and classical theories are no longer applicable.

In phase I, the EPATCS will be transferred and built in multiple advanced laser facilities. The platform will be transferred to the 0.5 PW femtosecond laser facility, which is commissioned in the Key Laboratory for Laser Plasmas at Shanghai Jiao Tong University. According to the matching condition of the LWFA, a low-energy dispersive electron beam with a maximum energy of less than 1 GeV can be generated stably. Another focused laser beam with an intensity of up to ${10}^{21}\ \mathrm{W}\cdot {\mathrm{cm}}^{-2}$ corresponds to a normalized intensity of ${a}_0\sim 20$ . Under this premise, the quantum parameter is ${\chi}_\mathrm{e}\sim 0.2$ . In this regime, electrons interacting with an intense laser field undergo nonlinear Compton scattering, with multiphoton scattering emerging as a significant characteristic. The emission of high-energy radiation results in significant energy loss for the electrons. Future scientific exploration based on this platform will focus on investigating radiation reaction effects, nonlinear Compton scattering, etc.

In phase II, the platform will be transferred to PW-level laser facilities, for instance, on a 1 PW laser facility at the Synergetic Extreme Condition User Facility (SECUF) of the Institute of Physics in the Chinese Academy of Sciences. It can contribute to studies of the classical radiation-dominated regime (CRDR)^[54^,55^] with normalized amplitude ${a}_0\sim 40$ . It can also be transferred to a 2.5 PW Ti:sapphire femtosecond laser system at the Tsung-Dao Lee Institute (TDLI), with a normalized amplitude ${a}_0$ that can be up to 60. Meanwhile, a high-quality electron beam with a central energy of 2 GeV, corresponding to $\gamma \approx 4000$ , can be generated using an intense PW laser. At this point, the quantum parameter is ${\chi}_\mathrm{e}\sim 1.4$ , which allows the radiation reaction phenomenon to be observed more directly. Independently, high-precision spatiotemporal synchronization of the EPATCS is crucial to validate and explore the locally monochromatic approximation (LMA)^[56^] and the local-constant-field approximation (LCFA)^[57^–61^]. As the phenomenon becomes more pronounced, it will deepen our understanding of the fundamental principle of SF-QED.

In phase III, the EPATCS could be transferred to 10 PW laser facilities, such as SULF^[62^], SEL^[63^], APOLLON^[64^], ELI^[65^,66^] and EP-OPAL^[67^]. Considering that a stable electron energy of multi-GeV could be generated, the normalized amplitude ${a}_0$ of the scattering laser would be up to 200, when the laser beam is focused to the diffraction limit, and the quantum parameter could be higher than 4. When ${a}_0\gtrsim 137$ and ${\chi}_\mathrm{e}\gtrsim 1$ , the radiation reaction in each photon emission is generally substantial and is fully accounted for within the context of SF-QED^[68^,69^]. Under such conditions, quantum effects will become consequential, which will be conducive to researching the experimental phenomena in the quantum radiation-dominated regime (QRDR)^[54^,69^,70^]. Furthermore, in this scenario, when the electron collides with the laser, the resulting high-energy gamma photons interact again with the intense laser. This interaction allows for effective detection of pair production, providing experimental evidence for the nonlinear Breit–Wheeler process^[29^,31^]. This will deepen our understanding of SF-QED processes and further refine the SF-QED theories and experimental validation. The relevant international experimental progress and proposals are summarized in Figure 6.

The development of an all-optical Thomson/Compton scattering platform, the EPATCS, featuring versatile parameter tunability, harnesses the exceptional benefits of LWFA and super-intense ultrafast lasers. We successfully demonstrate the capability of the EPATCS by providing precise control over the parameters of the X-rays of ICS, including the energy spectrum, polarization state and other parameters of high-energy radiation. With the ongoing development of super-intense ultrafast laser facilities, we aim to explore SF-QED in depth under various physical regimes, including radiation damping, nonlinear Compton scattering and nonlinear Breit–Wheeler electron–positron pair production.